A problem exists in the measure of solids or particles in suspension when the concentration of such particles is low and the size of the particles is small (e.g. 0.05 to 2 μm) in size, as in the detection of an integrity fault or “break-through” in the micro-filtration process of drinking water. A typical filter or cartridge used in micro-filtration is composed of a plethora of individual fibers through which the unfiltered or “unfinished” water is passed. In order to assess the integrity of the filter, the effluent water of the filtering process is monitored in comparison to an established limit of turbidity in drinking water.
It is desirable that the turbidimeter be of low volume so that response to a change in the amount of solids in the effluent is rapid and that sample volume used for monitoring purposes is insignificantly small in comparison to the volume of filtered suspension. Another desirable attribute of a turbidimeter is an ability to generate a detectable change in signal due to an incremental corresponding change in the amount of solids in the suspension. A conventional turbidimeter cannot achieve these requirements.
A conventional turbidimeter measures the concentration of solids in suspension by means of projecting a beam of light through a medium and measuring the amount of light scattered by the suspended solids. The lower limit of detection of this method is determined by the self-generated signal of the detector means. A conventional turbidimeter can approach the lowest limit of detection only when the amount of light that reaches the detection means not due to light scattered from the suspended solids is sufficiently reduced to below that of the self-generated signal of the detector means or noise level of the detector. Light that reaches the detector and generates a signal not associated with solids in the suspension medium is stray light. A signal generated by the detector due to stray light is not distinguishable from the signal which is caused by the suspended solids, thus the lower limit of detection is increased to a level greater than that caused by stray light.
As the volume of a conventional turbidimeter is reduced, the signal level of the detection means due to stray light increases due to diffuse reflections of the internal surfaces and air bubbles both in suspension and on the internal surfaces of the turbidimeter. Stray light that is diffusely reflected from internal surfaces is caused by imperfection in the quality of the surfaces and/or by the presence of air bubbles or other particles that form or adhere to the internal surfaces. Diffusely reflected light can illuminate other internal surfaces. When an internal surface is illuminated by direct or non-direct means and that surface falls within the field of view of the detector means, a signal is generated that increases the detection limit of the turbidimeter making it insensitive to small changes in the turbidity of the suspension. Air bubbles present in the suspension media have a similar effect in increasing the detection limit by diffusely reflecting and refracting light within the internal volume of the turbidimeter. Air bubbles efficiently refract and reflect light in all directions due to the spherical nature of the air bubble and due to differences of the refractive index of air and the refractive index of the suspension media in which the bubbles reside.
Another cause of poor detection of solids in suspension is illumination of the sample that is unequal to the volume of the sample that falls within the field of view of the detector. Illumination that falls outside of the field of view of the detector does not contribute to the signal generated from the solids in the suspension, and the signal level is decreased due to loss in irradiance of the sample. If the field of view of the detector is greater than the volume of the sample illuminated by the source beam, an increased susceptibility to stray light will ensue.
The turbidimeter of the present invention incorporates a novel arrangement of internal surfaces, optical surfaces, and optical restrictions to the field of view of both illumination and detection means to significantly improve the lower detection limit of a turbidimeter by reducing the detected signal not due to solids in suspension.